It was assumed that in vertebrates melatonin is only produced in the pineal gland and retina. However, evidence has accumulated that melatonin is widely detectable throughout Nature, being found in vertebrates, invertebrates, plants, unicellular eukaryotes, algae and even bacteria. We have documented that melatonin is produced and metabolized by the mammalian skin, where it can permeate to all cellular compartments including mitochondria. Melatonin (an amphilic methoxyindole) has pleiotropic bioactivities, as neurotransmitter, hormone, cytokine and biological response modifier, which are mediated by interactions with high affinity receptors. Melatonin is also a broad-spectrum antioxidant and acting via receptor-independent manner it activates cytoprotective pathways and at high concentrations it attenuates or counteracts the oxidative stress. These antioxidant effects are also shared by its metabolite, N1-acetyl-N2-formyl-5- methoxykynuramine (AFMK), phylogenetically the oldest product of melatonin metabolism. Thus, melatonin exhibits a number of properties that are extremely useful for the integumental stress-response systems. Accordingly, we have proposed that the cutaneous melatoninergic system enables for cell-specific selectivity through intra-, auto- or paracrine mechanisms, which allow locally counteracting or buffering both environmental and endogenous stressors to maintain skin integrity, independent of circadian pineal melatonin secretion. Such functions should include activation of cytoprotective and anti-apoptotic pathways, and antimutagenic, antigenotoxic and anticancerogenic activites. Melatonin bioactivity would depend on preexistent local degradative pathways and the intensity of exposure to ultraviolet radiation (UVR), e.g., melatonin can be converted non-enzymatically to AFMK through action of reactive oxygen species or UVB. Therefore, we are proposing: 1. To define the protective role of melatonin in human epidermis using epidermal keratinocytes and melanocytes cultured in vitro and human skin explants histocultured ex-vivo; 2. To define the role of melatonin in regulation of melanogenesis; 3. To characterize the pathway of melatonin degradation in cultured human keratinocytes and melanocytes. Realization of these aims will define whether melatonin can be exploited therapeutically, either as a general skin survival factor with anti- genotoxic properties or as a guardian of genome and cellular integrity. Thus, it may be used as a component of sunscreens or in the treatment of pigmentary diseases (e.g. vitiligo or hyperpigmentation), proliferative processes (including precancerous states, epidermal cancer or even melanoma), UVB-induced pathology, inflammatory dermatoses and skin aging.